In manufacturing, conveyors are commonly used to transport workpieces from workstation to workstation where manufacturing operations are performed on each workpiece. Typically, these conveyors consist of a pair of laterally spaced apart chains in closed loops that are carried by sprockets on a shaft at each end of the loop. Each chain is slidably guided and supported by a support rail attached to the conveyor frame. In operation, the chains are operably connected to several pallets, usually enough to fill the conveyor, carrying workpieces to transport the workpieces from station to station. In this fashion, the conveyor also serves as a storage system for the pallets and workpieces.
A long-standing problem with chain and even belt-type conveyors that are long, heavily loaded, and/or operate at slow speeds is the phenomena known as "surging" which generally manifests itself in the conveyor moving in a visually noticeable, jerking manner. This surging of the conveyor is highly undesirable since it can cause excessive wear and premature failure of the chains, sprockets, and other components of the conveyor. It is especially intolerable where a slow and stable assembly line speed is required for safe transportation of pallets and workpieces along an assembly line.
Surging appears to be caused by an operating condition known as "stick-slip" where at least one portion of a conveyor chain alternately decelerates and accelerates due to friction between the chain and guide rail and because the chain behaves like a massive spring alternately storing and releasing energy as the chain sticks and slips. Stick-slip in conveyors has been more specifically referred to as "harmonic oscillation stick-slip" because during surging it has been found that a conveyor chain will surge at a specific frequency which varies only slightly with changing lubrication conditions and the location of the load on a given conveyor.
To prevent surging, designers have generally limited conveyor lengths to less than 30 feet and chain speeds to greater than 50 feet per minute. As a result of these limitations, long manufacturing lines requiring a payload of pallets and workpieces to be transported at a relatively slow speed over long distances typically have used a series of short conveyors with expensive load-unload transfer mechanisms between conveyors to transfer the pallets and workpieces between each adjacent pair of conveyors. This solution to preventing surging is highly undesirable because it dramatically increases the cost and complexity of the conveyor line while decreasing its reliability.
It is also not uncommon for a conveyor that has operated without incident in the past to encounter surging, especially where operating conditions such as speed, payload or payload position have changed. Common solutions to eliminate or prevent surging in existing conveyors have been to reduce the payload transported by the conveyor or suitably increase the speed of the conveyor. Since a conveyor in this situation is usually preferably driven at close to the optimum speed at which manufacturing operations at each station can be performed, the conveyor speed is increased and/or the payload is typically reduced to avoid surging. Unfortunately, where the conveyor payload has been reduced by lessening the number of pallets and workpieces transported by the conveyor, the efficiency of the assembly line is reduced and manufacturing costs are increased.
To maintain assembly line efficiency while preventing or eliminating surging, a number of other post-installation solutions have been proposed and implemented on conveyors with little success. For example, one previously used solution has been to substitute a bearing roller chain for each roller chain commonly used in these conveyors. Although, this type of chain can allow more stable conveyor operation at a lower speed, it is considerably more expensive than conventional roller chain and is also susceptible to surging at extremely low conveyor speeds and/or long lengths.
Another solution proposed and tried with extremely limited success has been to increase conveyor chain tension. Although in some instances increasing chain tension can prevent the onset of surging, it does not prevent surging under most operating conditions. It has been found that adjusting chain tension successfully prevents surging only for a relatively narrow band of unchanging operating conditions. For example, although a particular preset chain tension may successfully prevent the onset of surging when the conveyor is transporting a specific number of pallets, surging frequently returns should the payload change or the position of the payload shift. This solution, therefore, is impractical since conveyor load and position typically do not remain unchanged during actual operating conditions.
A still further proposed solution has been to add a brake to the tail end of the conveyor opposite the drive shaft to continuously dissipate energy for preventing surging. Unfortunately, while brakes have been found to prevent the onset of surging under an extremely narrow range of conveyor operating conditions, they also do not consistently prevent surging under the wide variety of conditions generally encountered during actual conveyor operation.
Another unrecognized problem contributing to surging in over-under conveyors having a pair of spaced apart chains is the presence of a second, higher frequency, vibration during conveyor operation. In a conveyor also experiencing surging, this vibration can constructively and destructively interfere with the vibration associated with surging, increasing the amplitude of surging in one portion of one or both chains and lessening it another portion further increasing wear and tear on the conveyor.